The present invention relates to integrated circuits, and, more particularly, to integrated circuits with reduced leakage current.
A current trend in integrated chip technology is to reduce operating power (i.e., supply voltage times supply current) as low as possible. Indeed, as the physical size of integrated circuit components, such as transistors, has been scaled down, so has the supply voltage that powers the integrated circuit. The supply voltage must be scaled as dimensions are scaled to avoid various kinds of field-induced breakdown. To maintain high performance (i.e., switching speed of the transistors) of the reduced-power integrated circuits, process engineers have tended to use transistors having a reduced threshold voltage—i.e., the cutoff voltage level at which point the transistors switch from “on” to “off” and vice versa—to implement the switching logic of the integrated circuit. While lower threshold voltages maintain fast switching speeds of the transistors in the face of ever-decreasing supply voltages, the importance of “leakage” current increases as the threshold voltage becomes increasingly smaller.
For small-geometry integrated circuits—circuits wherein transistors having relatively low threshold voltages comprise the circuit's switching logic—a large component of the leakage current is subthreshold leakage. Subthreshold leakage is current leakage between the source and drain of the transistor when the transistor is “off.” For various applications, the leakage current is relatively negligible when the device is in “active mode”, but leakage current is less tolerable when the device spends a significant amount of time in “standby mode”, in which case the leakage current causes power to be wasted with no useful operation being performed. For example, for applications like cellular telephones and laptop computers, which are in standby mode much of the time, the battery life can be improved by reducing the leakage current when the device is in standby mode.
Subthreshold leakage current is particularly acute in integrated circuits comprised primarily of low threshold transistors because low threshold transistors have relatively low resistance as compared to high threshold transistors when they are in their “off” mode. In contrast, high threshold transistors have relatively high resistance when they are in their “off” mode. While it would be possible to decrease the leakage current of an integrated circuit simply by using transistors having relatively high threshold voltages throughout the switching logic of the circuit, the use of such high threshold transistors would unacceptably limit the switching speeds of the transistors in low voltage circuits. As a result, the overall performance (switching speed) of the integrated circuit would suffer. Therefore, the inventors hereof have identified a need for an improved circuit and method for reducing leakage current in integrated circuits comprised of low threshold voltage transistors.